The present invention relates to a method of flattening a surface of a semiconductor film formed by various film forming techniques, such as CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), paste printing, and burning.
Conventionally well-known semiconductor materials that are highly sensitive to x-rays and other kinds of radiation include CdTe and CdZnTe as described in xe2x80x9cCdTe Radiation Detector And Its Recent Developmentsxe2x80x9d (Applied Physics, Vol 65. No. 10, pp.1047-1051, issued in October 1996), for example. Also, in some cases, CdTe is used in CdS/CdTe thin film photovoltaic cells as described in xe2x80x9cMatsushita Gihoxe2x80x9d (Matsushita Technical Journal Vol.44 No.4 pp.477-480 issued in August 1998).
To manufacture a radiation detector element or a photovoltaic cell that incorporates a large substrate, using the aforementioned semiconductor materials, a polycrystalline film needs to be formed of CdTe, CdZnTe, etc. on a substrate made of, for example, glass by MOCVD (Metal Organic Chemical Vapor Deposition), proximate sublimation, paste printing, burning, or other techniques.
However, it is known that the surface of a thick polycrystalline CdTe or CdZnTe film formed by these techniques has irregularities developing from crystal particles in a random manner. The phenomenon is observed when a film is formed of CdTe, CdZnTe, etc. regardless of which film forming technique is used of MOCVD, proximate sublimation, paste printing, or burning.
The irregularity on the film surface can, of course, be reduced by controlling the size of crystals through optimization of film forming conditions, but only to some extent: if a film needs to be formed in a thickness of several hundred xcexcm, it is difficult to restrain the irregularity on the film surface to a several xcexcm or lower level. Especially, when the polycrystalline CdTe or CdznTe film is used in an x-ray detector element, since the film needs to be 100 xcexcm thick or even thicker with the absorption efficiency of x-rays taken into consideration, the development of the foregoing irregularity is inevitable.
As in the foregoing, those irregularities of a magnitude of several xcexcm that develop on the surface of a semiconductor film after the formation of the film are a cause for various problems; for example, an electric charge inhibition layer formed on the semiconductor film can only insufficiently establish a junction to the semiconductor film at the interface of the two, and therefore fails to inhibit electric charges in a desirable manner, or it becomes difficult to perform fine fabrication on a electric charge inhibition layer and electrode layer formed on the surface of a semiconductor film. These problems make it difficult to manufacture radiation detector elements (e.g., two-dimensional image detectors) and light detector elements (e.g., photovoltaic cells) with satisfactory levels in performance and reliability.
The present invention has an object to offer a method of flattening a surface of a semiconductor film by easily eliminating irregularities from the surface of a semiconductor film.
In order to achieve the object, the method of flattening a surface of a semiconductor film in accordance with the present invention is characterized in that it includes the step of bombarding an irregular surface of a semiconductor film with abrasive particles so as to grind the irregular surface.
Irregularities are found in some cases on the surface of a semiconductor film just formed because the crystal particles constituting the semiconductor film are present on the surface in a random manner. That irregular semiconductor film, if used as it is in a radiation detector element, fails to deliver satisfactory performance and reliability; therefore, such irregularity should be minimized.
To achieve the task, conventionally, conditions are optimized in the formation of a semiconductor film, so as to control the crystal particles in terms of their diameters and eventually to reduce the irregularity. The conventional method successfully reduces the irregularity, but only to some extent: the dimensions of irregularities are still larger than the diameters of crystal particles. This is not practical for realization of a higher level of fine fabrication.
Meanwhile, the irregular surface of a semiconductor film processed by the foregoing method in accordance with the present invention is flattened as it is ground by bombarding abrasive particles. Since the surface of a semiconductor film is flattened through grinding, the dimensions of irregularities are readily reduced below the diameters of the crystal particles constituting the semiconductor film, and possibly to minimum levels. Further, only a simple device is required to bombard a semiconductor film with abrasive particles; therefore, the foregoing method of flattening can be incorporated in the manufacture of radiation detector elements and the like at small cost, and imparts increased reliability to the radiation detector elements manufactured.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.